Increasing concern over exposure to pesticides, nerve agents, and chemical and biological pollutants, along with more-frequent chemical and biological threats due to intentional or accidental release of toxic agents, necessitates the development of countermeasures that provide effective protection for military personnel and emergency responders. Currently, protective clothing systems, such as hazardous material (HAZMAT) suits or joint service lightweight integrated suit technology (JSLIST), are widely used to achieve full protection. These protective systems are based upon either full barrier protection through blocking contaminant permeation, or air-permeable adsorptive protective overgarments in which all the toxins are adsorbed on contact [Schreuder-Gibson, H. L.; Truong, Q.; Walker, J. E.; Owens, J. R.; Wander, J. D.; Jones, W. E. MRS Bulletin 2003, 28, 574]. High thermal loads from poor water-vapor-permeability and excess insulation as well as the weight and bulkiness of these protective fabrics and suits impair the wearer's performance. Recently, protective fabrics have been developed based on selectively-permeable membranes, which allow for permeation of water vapor while remaining resistant to the permeation of organic molecules [Wilusz, E.; Truong Q. T.; Rivin, D.; Kendrick, C. E. Polym. Mat. Sci. Eng. 1997, 77, 365]. The new generation of protective fabrics is envisioned not only to absorb or block toxic chemical and biological agents but also to detoxify them to reduce the risk of secondary contamination [Schreuder-Gibson, H. L.; Truong, Q.; Walker, J. E.; Owens, J. R.; Wander, J. D.; Jones, W. E. MRS Bulletin 2003, 28, 574].
Electrospinning is a fiber-forming process that employs electrostatic forces to stretch a jet of polymer solution or melt, producing continuous fibers with diameters ranging from micrometers down to several nanometers [Dzenis, Y. Science 2004, 304, 1917; Rutledge, G. C.; Fridrikh, S. V. Adv. Drug Delivery Rev. 2007, 59, 1384; Reneker, D. H.; Yarin, A. L. Polymer 2008, 49, 2387; Reneker, D. H.; Chun, I. Nanotechnology 1996, 7, 216; Greiner, A.; Wendorff, J. H. Angew. Chem. Int. Ed. 2007, 46, 5670; Li, D.; Xia, Y. Adv. Mater. 2004, 16, 1151; Ramakrishna, S.; Fujihara, K.; Teo, W. E.; Yong, Y.; Ma. Z. W.; Ramaseshan, R. Materials Today 2006, 9, 40; and Ramakrishna, S.; Fujihara, K.; Teo, W.-E.; Lim, T.-C.; Ma. Z. An Introduction to Electrospinning and Nanofibers, World Scientific Publishing Company: Singapore, 2005]. Electrospun nanofibers attract great interest in the materials science community due to their ease of processing, ease of functionalization, high surface area, light weight, breathability and flexibility. Electrospun nanofibers for chemical and biological protection are well-represented in the art [U.S. Pat. No. 7,445,799 (Sarangapani), hereby incorporated by reference in its entirety; and K. Graham, M. Gogins, H. Schreuder-Gibson, “Incorporation of Electrospun Nanofibers Into Functional Structures,” Presented at INTC 2003, sponsored by INDA, Association of the Nonwoven Fabrics Industry and TAPPI, Technical Association of the Pulp & Paper Industry, Sep. 15-18, 2003, Baltimore, Md.]. The ease of implementation as well as the remarkable properties of electrospun fiber mats, such as small fiber size, high specific surface area, high porosity and low fabric weight, have inspired the use of the electrospun fiber mats in a broad range of applications, including scaffolds in tissue engineering, composite materials, filters, sensors and energy storage devices [Ma, Z. W.; Kotaki, M.; Inai, R.; Ramakrishna, S. Tissue Eng. 2005, 11, 101; Roso, M.; Sundarrajan, S.; Pliszka, D.; Ramakrishna, S.; Modesti, M. Nanotechnology 2008, 19, 285707/1-285707/6; Yoon, K.; Hsiao, B. S.; Chu, B. J. Mater. Chem. 2008, 18, 5326; Thavasi, V.; Singh, G.; Ramakrishna, S. Energy Environ. Sci. 2008, 1, 205; and Kim, I. D.; Rothschild, A.; Lee, B. H.; Kim, D. Y.; Jo, S. M.; Tuller, H. L. Nano Lett. 2006, 6, 2009]. The potential for application of electrospun fiber mats in protective clothing was demonstrated by Schreuder-Gibson and coworkers [Gibson, P.; Schreuder-Gibson, H. L.; Rivin, D. Colloids Surf. A 2001, 187-188, 469; Gibson, P. W.; Schreuder-Gibson, H. L.; Rivin, D. AICHE Journal 1999, 45, 190; and Gibson, H. L.; Gibson, P.; Senecal, K.; Sennett, M.; Walker, J.; Yeomans, W.; Ziegler, D.; Tsai, P. P. J. Adv. Mater. 2002, 34, 44]. They showed that lightweight electrospun fabrics exhibit higher breathability than do barrier materials while displaying better airflow resistance and enhanced aerosol particle retention compared to current commercially available membranes. “Breathability” in this instance is defined as transmission of water vapor but not liquid water.
Breathable porous membranes such as Celgard® 2400, PAN/PET (a polyethyleneterephthalate fleece coated with a thin layer of polyacrylonitrile) and Isopore™ (an etched ion-track polycarbonate membrane) coated by alternating electrostatic adsorption of cationic and anionic compounds (polyelectrolytes and bolaamphiphiles) are known in the art [F. Van Ackern, L. Krasermann, B. Tieke, “Ultrathin membranes for gas separation and pervaporation prepared upon electrostatic self-assembly of polyelectrolytes,” Thin Solid Films 1998, 327-29, 762-766; and the International Conference on Organized Molecular Films No. 8, held in Pacific Grove, Calif., on Aug. 24, 1997], but they are not based on electrospun nanofibers.
However, Obendorf et al. showed that laminated fabrics with electrospun polypropylene fiber layers significantly limit the penetration of liquid pesticides while still maintaining good water vapor permeability [Lee, S.; Obendorf, S. K. J. Appl. Polym. Sci. 2006, 102, 3430]. The combination of high breathability and efficient barrier properties of electrospun fabrics makes them promising candidates for the next generation of protective clothing. Moreover, the high specific surface areas of electrospun fiber mats allow attachment of functional compounds to obtain chemical or biological detoxifying protective clothing. Ramakrishna et al. successfully electrospun fibers with a reactive compound, (3-carboxy-4-iodosobenzyl)oxy-β-cyclodextrin, and showed that these reactive fabrics can decompose paraoxon, an organophosphate pesticide [Ramaseshan, R.; Sundarrajan, S.; Liu, Y.; Barhate, R. S.; Lala, N. L.; Ramakrishna, S. Nanotechnology 2006, 17, 2947]. In another study, electrospun zinc titanate nanofibers were tested as reactive sorbents capable of detoxifying nerve and mustard agent simulants [Ramaseshan, R.; Ramakrishna, S. J. Am. Ceram. Soc. 2007, 90, 1836]. Various biocides such as silver nanoparticles, quaternary ammonium salts or their derivatives, compounds with biguanide groups, and N-halamine, have been incorporated into electrospun fiber membranes to serve as antimicrobial filters or to create a biological protective clothing [Lala, N. L.; Ramaseshan, R.; Bojun, L.; Sundarrajan, S.; Barhate, R. S.; Ying-jun, L.; Ramakrishna, S. Biotechnol. Bioeng. 2007, 97, 1357; Fu, G-D.; Yao, F.; Li, Z.; Li, X. J. Mater. Chem. 2008, 18, 859; Fan, L.; Du, Y.; Zhang, B.; Yang, J.; Zhou, J.; Kennedy, J. F. Carbohydr. Polym. 2006, 65, 447; Chen, L.; Bromberg, L.; Hatton, T. A.; Rutledge, G. C. Polymer 2008, 49, 1266; and Tan, K.; Obendorf, S. K. J. Membr. Sci. 2007, 305, 287].
Functionalization of electrospun nanofibers with antimicrobial functionality such as silver nanoparticles is also known in the art [N. L. Lala, R. Ramaseshan, L. Bojun, S. Sundarrajan, R. S. Barhate, L. Ying-jun, S. Ramakrishna, “Fabrication of nanofibers with antimicrobial functionality used as filters: protection against bacterial contaminants,” Biotechnol. Bioeng. 2007, 97 (6), 1357-1365]. However, silver nanoparticles, while being bactericidal and bacteriostatic through the action of silver ions slowly releasing into the environment, are not effective in killing a wide range of microorganisms on contact.
The use of small molecular weight, broad-range bactericides, such as chlorhexidine, for incorporation into nanofibers, either through physical enmeshment or covalent attachment, has also been disclosed [L. Chen, L. Bromberg, T. A. Hatton, G. C. Rutledge, “Electrospun cellulose acetate fibers containing chlorhexidine as a bactericide,” Polymer 2008, 49 (5), 1266-1275]. The resulting nanofibers are capable of killing bacteria on contact, but are unable to degrade organophosphorous esters, which are common pesticides and chemical warfare agents.
Chemical methods to counteract nerve agents and remediate organophosphate (OP) contamination by means of nanoparticles, polymers and nanofibers functionalized by α-nucleophilic agents are known [Bromberg, L.; Schreuder-Gibson, H.; Creasy, W. R.; McGarvey, D. J.; Fry, R. A.; Hatton, T. A. Ind. Eng. Chem. Res. 2009, 48, 1650; Bromberg, L.; Zhang, H.; Hatton, T. A. Chem. Mater. 2008, 20, 2001; Bromberg, L.; Hatton, T. A. Ind. Eng. Chem. Res. 2005, 44, 7991; Chen, L.; Bromberg, L.; Schreuder-Gibson, H.; Walker, J.; Hatton, T. A; Rutledge, G. C. J. Mater. Chem. 2009, 19, 2432; Chen, L.; Bromberg, L.; Hatton, T. A.; Rutledge, G. C. Polymer 2007, 48, 4675]. For example, fiber mats functionalized with α-nucleophilic oxime moieties were prepared by either electrospinning blends of polyacrylamidoxime (PAAO) and polyacrylonitrile (PAN) or surface oximation of prefabricated PAN fiber mats, and demonstrated to possess a pronounced capability to hydrolyze chemical nerve agent simulants in the presence of moisture. The fiber post-spin modification strategy has the advantages of higher surface density of oxime functional groups, enhanced reactivity and ease of implementation compared to the PAN/PAAO blending strategy.
Polycationic polymers, such as poly(N-vinylguanidine), are capable of both killing a broad range of bacteria and catalytically degrading organophosphate esters such as warfare agents [L. Bromberg, T. A. Hatton, “Poly(N-vinylguanidine): Characterization, and catalytic and bactericidal properties,” Polymer 2007, 48, 7490-7498], but are stably bound if adsorbed onto electrospun nanofibers.
Polyanionic polymers, such as poly(sodium hydroxamate)s and poly(sodium acrylamidoxime)s, are known in the art to degrade chemical warfare agents [L. Bromberg, H. Schreuder-Gibson, W. R. Creasy, D. J. McGarvey, R. A. Fry, T. A. Hatton, “Degradation of chemical warfare agents by reactive polymers,” Ind. Eng. Chem. Res. 2009, 48, 1650-1659], but would be separated from electrospun nanofibers if simply enmeshed, in aqueous milieu.
The layer-by-layer (LbL) electrostatic assembly technique offers another strategy of electrospun fiber surface functionalization. The LbL electrostatic assembly is a simple, versatile and inexpensive approach to generate functional multilayer thin film coatings on surfaces [Hammond, P. T. Adv. Mater. 2004, 16, 1271; Decher, G. Science 1997, 277, 1232]. The utilization of electrospun fiber mats as substrates for LbL-based functional coatings enhances the functioning of multi-layered coatings by significantly increasing the specific surface area of substrates [Wang, X.; Kim, Y-G.; Drew, C.; Ku, B-C.; Kumar, J.; Samuelson, L. A. Nano Lett. 2004, 4, 331; Yang, G.; Gong, J.; Yang, R.; Guo, H.; Wang, Y.; Liu, B.; Dong, S. Electrochem. Commun. 2006, 8, 790; Lee, J. A.; Krogman, K. C.; Ma, M.; Hill, R. M.; Hammond, P. T.; Rutledge, G. C. Adv. Mater. 2009, 21, 1252; and Krogman, K. C.; Lowery, J. L.; Zacharia, N. S.; Rutledge, G. C.; Hammond, P. T. Nat. Mater. 2009, 8, 512]. Wang et al. showed that a fluorescent probe LbL-assembled onto electrospun cellulose acetate membranes resulted in a dramatic increase in sensitivity of optical sensors. For the application in protective fabrics, Lee et al. demonstrated that electrospun fibers used as the substrate for titanium dioxide nanoparticle coatings increased the substrate surface area by 104 times compared to the flat film, which enhanced photo-catalytic decomposition of toxic industrial chemicals. In addition, Krogman et al. employed a newly-developed spray-assisted LbL assembly technique to functionalize electrospun nylon fibers with titanium dioxide nanoparticles for protective fabrics, in which they could obtain conformal coatings or bridge the surface voids by controlling the spraying conditions; in this regard, they could achieve LbL-functionalized fiber mats with improved photo-catalytic capability without sacrificing water vapor permeability or breathability of the electrospun fiber mats.